The developmental processes during neuromuscular junction formation and the mechanism of Ca entry in secretory cells were studied. 1) Changes in muscle membrane properties during acetylcholine receptor (AChR) accumulation and neurospecific interaction in Xenopus nerve-muscle cell culture: Properties of spontaneous synaptic potentials change as AChRs accumulate to the nerve contact area. Non-cholinegic neurons, dorsal root ganglion cells, do not form function synaptic contact nor cause AChR accumulation. Tadpole spinal cord explants form functional synaptic contacts with muscle cells but rarely cause AChR localization. Thus, the ability to induce AChR accumulation in the muscle cell seems to be restricted to a certain type of cholinergic neuron. 2. Neuromuscular junction formation between embryonic Xenopus neurons and rat myotubes: When Xenopus neurons were co-cultured with rat myotubes, they were found to form functional synaptic contacts but did not induce AChR accumulation. This result suggests that the rat myotube in culture is defective in exhibiting AchR accumulation. 3. Effect of tetrodotoxin on adrenaline secretion in the perfused rat adrenal medulla: Tetrodotoxin, a specific inhibitor of the Na action potential, decreased KCl or ACh-stimulated adrenaline release. The Na action potential, by facilitating the activation of voltage-dependent Ca channels, may play a regulatory role in catecholamine release.